The present invention relates to shield materials and structures for nuclear reactors which generate substantial amounts of fast neutron radiation during operation and more particularly to shield materials and structures for use in nuclear reactors of a type employed for example in power generation. In the design of nuclear reactors involving either fission or fusion reactions, provision must be made for the attenuation of escaping nuclear radiation by means of suitable shield structure. The shielding must be capable of moderating and absorbing the various types of radiation generated within the reactor. Normally the most significant types of radiation for which such shielding is required are primary neutrons and gamma rays originating within the core and secondary gamma rays produced by neutron interaction with materials external to the core such as reflector components, coolant or even materials within the shield itself.
The reactor shielding must of course provide irradiation protection for personnel in the vicinity of the reactor. However, shielding is also necessary for various other functions. For example, radiation from the reactor may interfere with the satisfactory functioning of instruments employed in various operations and control aspects of the reactor and associated components such as vapor generators and the like. Furthermore, radiation encountering the shield materials may produce internal heating and tend to cause radiation damage in various components of the shield.
Accordingly, the selection of materials and the structural design of a reactor shield is primarily dependent upon the purpose or application for the reactor itself. For example, a reactor employed to generate sufficient energy for the operation of a power station may be considered as a stationary system with the shield being a substantial or massive structure. Accordingly, the materials and structural design of the shield must be selected to moderate and absorb radiation from the reactor as well as to provide structural support for itself and related components of the reactor while also being designed to permit adequate heat transfer in order to maintain temperature levels of the shield itself within satisfactory limits.
The selection of material and structure of the shield is also largely dependent upon the specific type of radiation generated by the reactor during operation. As noted above, the present invention is contemplated for use in connection with a gas cooled, fast neutronic reactor where fast neutrons are produced in a fission process. However, it will be apparent from the following description that the invention is equally applicable for use in a variety of reactors involving substantial amounts of fast neutron generation, either from fission or fusion processes.
In a neutronic reactor of the type referred to above, neutrons generated within the reactor core may experience scattering collisions, mainly elastic, as a result of which their energy is decreased. Thereafter, they may be absorbed by various materials within the reactor core or they may escape. Depending upon the design of the reactor, neutron captures leading to fission reactions within the reactor tend to occur within specific energy ranges. If most of the fission results from the capture of thermal neutrons or neutrons of an intermediate energy range, the system may be referred to as a thermal reactor or intermediate reactor. However, if the fission process results primarily from the capture of fast neutrons, the system is generally referred to as a fast reactor.
Details concerning these various types of reactors are generally well known in the art. For purposes of the present invention, it is sufficient as noted above to indicate that the invention is particularly directed toward nuclear reactors involving substantial amounts of fast neutron generation which must be contained by suitable shielding. It will be obvious that such a reactor may also generate other types of irradiation such as thermal neutrons and gamma rays for example.
For purposes of the present invention, fast neutrons are defined as having a flux or energy of about 70 keV or greater.
In selecting the materials and structural design for suitable shielding in such reactors, it should also be kept in mind that the shielding involves a substantial volume of the overall reactor. For this reason and because of the need to assure adequate containment while avoiding excessive temperatures or radiation damage over prolonged periods of operation, efficiency and economics of a reactor may be due in large part to design and material components of the shield.
Accordingly, there has been found to remain a need for nuclear reactor shields including suitable materials and structure which permit efficient containment of reactor cores generating substantial quantities of fast neutrons.